The present invention relates to stable foams having a controlled fine air bubble size distribution and to edible products prepared therefrom having a low fat content. Particularly interesting products prepared from such foams include ice creams and related frozen products.
The manufacture of finely dispersed gas bubbles in a continuous liquid or semi solid fluid phase either denoted as gas dispersions for gas volume fractions below about 10-15%, or as foams for gas volume fractions higher than about 15-20% is of major interest in particular in the food, pharmaceutical, cosmetics, ceramics and building material industries. The gas fraction in related products of these industries has a strong impact on the physical parameters like density, rheology, thermal conductivity and compressibility and related application properties. In the area of foods, aeration of liquid to semi-solid systems adds value with respect to consistency and related perception/sensory properties like creaminess, softness and smoothness as well as improved shape retention and de-mixing stability. For specific food systems like frozen deserts or ice cream the strongly reduced thermal conductivity is another major stability factor protecting the product from quickly melting; e.g. due to thermal shocks applied in the “cooling chain” from the store to the consumer's refrigerator. The strong increase of inner interface may also give access to new area for adsorption and fixation/stabilization of functional/techno-functional molecules such as flavor and/or nutritionally active compounds.
In conventional frozen and aerated water-based ice slurries of the ice cream type, the typically important sensory properties like scoop ability, creaminess, smoothness, shape retention during melting and heat shock stability are determined by an interplay of the three disperse phases: air cells/bubbles, fat globules/fat globule agglomerates and water ice crystals within characteristic size ranges and volume fractions of these disperse components as shown for example in Table 1.
TABLE 1Size and volume fraction ranges of dispersephases in conventional ice creamfat globulegas/air cellsagglomerateswater ice crystalsMean diameter25-352-10050-60X50.0/μmvolume fraction/50-608-15 40-50% vol.
Well-stabilized small air cells are mainly responsible for the creaminess and smooth texture sensation during the melting of the ice cream in a consumer's mouth. Smaller air cells/foam structure in the melted state during shear treatment between tongue and palate results in a more pronounced perception of creaminess. Smaller air cell size also supports longer shelf life of frozen ice cream systems due to increased steric hindrance for ice crystal growth. At constant gas volume fraction a higher number of smaller air cells generates a larger total gas interface area and thus reduced thickness of lamellae formed between the air cells by the continuous watery fluid phase. This restricts ice crystal growth within these lamellae. Another but less pronounced direct contribution to creaminess is derived from medium sized fat globule agglomerates below 20-30 micron in diameter. When the fat globule aggregates get larger than about 30-50 microns, the creamy sensation turns into a buttery, fatty mouth feel.
The scoop ability of frozen, aerated slurries like ice creams is mainly related to the ice crystal structure, in particular the ice crystal size and their interconnectivity. Scoop ability is the most relevant quality characteristic of ice cream in the low temperature range between −20° C. and −15° C.
In conventional ice cream manufacture partial freezing is done in continuous or batch freezers, having cooled scraped surface heat exchangers, down to outlet temperatures of about −5° C. Then the ice cream slurry is filled into cups or formed at the outlet of extrusion dies. Thereafter the products are hardened in freezing tunnels with coolant air temperatures of around −40° C. until a product core temperature of about −20° C. is reached. Then the products are stored and/or distributed. After pre-freezing of conventional ice cream recipes in the ice cream freezer, about 40-45% of the freezable water is frozen as water ice crystals. Another fraction of about 55-60% of the freezable water is still liquid due to freezing point depression in the watery solution concentrated in sugars, polysaccharides and proteins. Most of this watery fraction freezes during further cooling in the hardening tunnel. In this hardening step, the ice cream is in the state of rest. Consequently the additionally frozen water crystallizes at the surfaces of the existing ice crystals, causing their growth from about 20 microns to 50 microns and larger. Some of the ice crystals interconnect to form a three dimensional ice crystal network. When such networks are formed, the ice cream behaves like a solid body and its scoop ability diminishes.
Certain patents such as U.S. Pat. Nos. 5,620,732; 6,436,460; 6,491,960; 6,565,908 disclose the restricting of ice crystal growth during cooling/hardening by the use of antifreeze proteins. This is also expected to have a positive impact on the ice crystal connectivity with respect to improved scoop ability.
U.S. Pat. Nos. 6,558,729, 5,215,777, 6,511,694 and 6,010,734 disclose the use of other specific ingredients like low melting vegetable fat, polyol fatty acid polyesters or specific sugars like sucrose/maltose mixtures to soften the related ice cream products, thus improving scoop ability and creaminess.
U.S. Pat. Nos. 5,345,781, 5,713,209, 5,919,510, 6,228,412 and RE36,390 disclose specific processing equipment, mostly single or twin screw continuous freezing extruders, to refine the ice cream microstructure (air cells, ice crystals and fat globule agglomerates) by using high viscous friction forces acting at the typically very low processing temperatures of 10° C. to −15° C. and thus improving the texture and stability properties.
Other publications disclose the use of mesomorphic surfactant phases with a premix having surfactants and water being prepared at specified temperature to provide a continuous lamellar phase. These documents include European patent application 753,995 and PCT publication WO95/35035. Another approach that discloses the use of mesomorphic phases of edible surfactant as structuring agents and/or fat substitutes can be found in U.S. Pat. No. 6,368,652, European patent application 558,523 and PCT publication WO92/09209.
PCT publication WO2005/013713 discloses an ice confection having at least 2% by weight fat and its manufacturing process, where some of all of fat are present as oil bodies.
Despite these disclosures, however, there remains a need for a process to form iced foams or iced confections that when frozen do not undergo pronounced gas bubble enlargement and its associated generation of pronounced solid body behavior or iciness.
Furthermore, novel aeration techniques to address the above need remain lacking. For example, industrial membrane based aeration technology is still rather new. Known conventional aeration or whipping of liquid fluid systems is commonly carried out using rotor/stator dispersive mixing devices in turbulent flow fields under very high energy input rate conditions.
Membrane based dispersing procedures are known in the area of liquid/liquid dispersing (emulsification) using static membrane modules in which the detachment of disperse liquid droplets is caused by membrane overflow with the continuous liquid phase. However this means that the forces or stresses supporting drop detachment are directly coupled with the volume flow rate of the continuous fluid phase. This is certainly not acceptable for the manufacture of related emulsion or dispersion systems if changes in volume flow rate would also impact on the drop size distribution of the disperse phase thus changing related system properties.
First attempts in membrane foaming have also been introduced using static membrane devices with the same type of problems as described for the liquid/liquid dispersion processing above, however with more pronounced problems concerning the generation of small bubbles in particular at higher gas volume fractions (>30-40%). This may be based on a well known physical relationship, described by the so-called critical Capillary Number (Cac). The major type of flow generated in the vicinity (i.e., Prandt1 boundary layer) of an overflow static membrane is shear flow. In shear flow the critical Capillary Number is a strong function of the viscosity ratio of disperse and continuous phases (ηdisperse/ηcontinuous). In particular for very low viscosity ratio in the range of ≦10−3-10−4 representing foam systems, Cac can reach values larger than about 10-30. The reason is that in spite of easy and large deformation of air bubbles in sheared liquids, there is no efficient break up, or in other words, the critical bubble deformation is strongly increasing with decreasing viscosity ratio. At very high volume flow rates turbulent flow conditions are reached with improved bubble dispersion. This is not satisfactory, however, with regard to bubble size and narrow bubble size distribution width. Even in the turbulent flow domain a laminar Prandt1 layer exists in the vicinity of the walls, thus limiting the turbulent dispersing mechanism.
Recently a rotating membrane device has been introduced for liquid/liquid dispersing showing the high potential of improved drop dispersing in particular with respect to small and narrowly size distributed droplets, but this device has not been used for gas dispersing or foaming. This is likely due to the problems related to the difficult gas bubble break up in shear dominated laminar flow described above, as well as due to the high density difference between the two phases which makes the process in rotational, particularly laminar flow fields, even more difficult. The gas phase having less than one percent of the liquid density tends to separate towards smaller radii (equivalent to lower centrifugal pressure) in the centrifugal force field acting in laminar rotational flows without flow-related disturbance. Such fundamental problems remain unsolved.
German patent application DE 101 27 075 discloses a rotating membrane device for the manufacture of emulsion systems. This device is not suitable, however, for the generation of finely dispersed homogeneous gas dispersions or foams due to the large radial dimensions of the dispersing gaps formed between the membrane modules and the housing, which would strongly support the de-mixing of the phases at higher rotational velocity required for the refinement of the gas bubbles.
PCT publications WO 2004/30799 and WO 01/45830 describe similar membrane devices for emulsion production with identical problems to those of gas dispersions or foams that were previously mentioned.
There is therefore a need for a novel aeration device and method to enable the formation of a low fat frozen foam product that when frozen does not form large gas bubbles or interconnected ice crystals and their subsequent solid body behavior. There is also a need for products that contain such a novel foam.